1. The Field of the Invention
The present invention relates to a control apparatus and method for an electric power steering system that transmits mechanical power from an electric motor to a steering system of a vehicle to reduce a required steering torque, in particular, a method and apparatus for automatically compensating for a change in steering characteristics of vehicle-mounted electric power steering system over time due to component deterioration with age to maintain input-output characteristics of the electric power steering system.
2. Description of the Related Art
An electric power steering system includes an electric motor to assist steering effort of an operator of a vehicle in a manner where so-called assist steering torque that is a driving force is applied from the electric motor in response to a steering operation of the operator under a control of a control device. A conventional electric power steering system determines the assist steering torque based on a steering torque applied from the operator of the vehicle to a steering shaft connected to a steering wheel and controls an output torque of an electric motor connected to a steering system based on the assist steering torque.
Steering feeling of the operator is determined by steering characteristics which are expressed by relationships between the steering torque applied from the operator of the vehicle and the output torque of the electric motor for assisting the steering operation of the operator, which is stored as a map in a control device of the electric power steering system. In general, the steering characteristics have been tuned up and optimized before the vehicle provided with the electric power steering system is delivered from a vendor to a user. However, long use of the electric power steering system causes a deterioration of the steering feeling of the operator because mechanical characteristics of the steering system such as a viscous friction coefficient of the steering shaft, a shaft of the electric motor, and the like, are changed.
Yasuda discloses in Japanese Laid-Open Patent Application No. 2002-293257 and in the corresponding U.S. Pat. No. 6,594,569 a device and method for setting steering characteristics of an electric power steering system. In this device, an operator of a vehicle providing with the device can observe a steering characteristics diagram that, for example, shows a relationship between a steering torque applied by the operator and an target value of motor current which is to be supplied to an electric motor of the electric power steering system to assist the steering effort of the operator, a relationship between an angle of a steering wheel and the target value of the motor current, and a relationship between an angular velocity of a steering wheel and the target value of the motor current, on a diagram display screen during operating the vehicle. The steering characteristics diagram is stored as an assist map in a memory of the electric power steering system. Further, the operator can reset, alter, and compile the steering characteristics diagram to vary the steering characteristics and steering feeling. This compilation of the steering characteristics diagram would be performed in order to achieve the optimum steering feeling in accordance with a vehicle model, operator's preferences, driving conditions of the vehicle such as weight of a load carried by the vehicle used hours of the electric power steering system. In particular, it is possible to reset the steering characteristics to return to a default characteristics which is set before the vehicle provided with the electric power steering system is delivered from a vendor to a user.
However, in this device the operator needs to manually alter the steering characteristics diagram to adjust the steering characteristics to favorable ones of the operator by himself. There may be an operator who cannot easily and quickly operate the compilation of the steering characteristics diagram because the operator has not been familiarized with operation of the compilation. Further, it is not easy to fine-tune the steering characteristics to reset the steering characteristics diagram for obtaining the default characteristics.
Further, Kifuku discloses in Japanese Laid-Open Patent Application No. 2000-103349 and in the corresponding U.S. Pat. No. 6,876,910 a vehicle-mounted electric power steering system in which an electric motor assists steering torque to improve steering feeling of an operator of a vehicle by estimating static friction, Coulomb friction, viscous friction, and inertia of the power steering system and compensating for these frictions and the inertia.
The electric power steering system includes a separately driven DC motor that generates steering assist force for assisting steering torque of an operator by his operation of a steering wheel in response to an input electric current Iin. The steering assist force is determined based on a steering torque applied to a steering shaft connected to a steering wheel, speed of the vehicle, angular velocity of the motor, and an angular acceleration of the motor. The angular velocity and the angular acceleration of the motor are estimated based on a motor current that flows in the motor and an applied electric voltage. The input electric current Iin of the motor is obtained by summing a static friction compensation current If, a Coulomb friction compensation current IC, a viscous friction compensation current Id, an inertia compensation current Ij, and an assist current Is together.
The static friction compensation current If is computed based on an estimated value of static friction of the power steering system that is estimated from a steering torque inputted from the operator to the steering system. The estimated value of the static friction is estimated based on a fact that the static friction influences on steering feeling of the operator, for example, an uncomfortable feeling in which the operator feels as if the steering wheel is stuck when the operator starts to operate the steering wheel especially around a neutral position of the steering wheel. Hence, the estimated value of the static friction is computed from change in the steering torque over time. The Coulomb friction compensation current IC has a constant value when the angular velocity of the motor exceeds a predetermined value. The viscous friction compensation current Id is linearly proportional to the angular velocity of the motor. The inertia compensation current Ij is linearly proportional to the angular acceleration of the motor. The assist current Is is computed based on the speed of the vehicle and the steering torque inputted by the operator of the vehicle to the steering wheel.
However, the constant value of the Coulomb friction compensation current IC, relationship between the viscous friction compensation current Id, and the angular acceleration of the motor, relationship between the inertia compensation current Ij and the angular acceleration of the motor, and relationships between the assist current Ix and each of the speed of the vehicle and the steering torque, are predetermined so that variations of values of the above mentioned frictions and the inertia of the steering system due to mechanical deteriorations of constituents of the steering system caused by long use of the steering system haven not been taken into consideration. Further, only the value of the static friction is explicitly estimated. Hence, explicit values of the Coulomb friction and the viscous friction are not obtained. Therefore, steering feeling of the operator is impaired when mechanical characteristics of the steering system such as the value of the viscous friction of the steering shaft, a shaft of the electric motor, and the like, are changed.
Further, even if techniques of Yasuda would be combined with those of Kifuku, an apparatus that automatically compensates for a change in steering characteristics of vehicle-mounted electric power steering system over time due to component deterioration with age to maintain optimum values of parameters of the electric power steering system can not be currently obtained.
Therefore, it is desired a method and apparatus for compensating a change in steering characteristics of vehicle-mounted electric power steering system over time due to component deterioration with age to keep comfortable steering feeling of an operator even if the electric power steering system would be used for long time.